design: Fix buttons in dark mode

Style buttons in dark mode to use the dark mode link
color, except for primary buttons which should use
the same button styles as light mode. Remove text
underline from buttons on hover. Add the primary
button highlight color from the OOUI demos.

Bug: T287498
Change-Id: Ifed7aab32ef4487a1c8053199319c6c8c4d8c286